BOSTON (CNN) -- "You're in Boston. You know stuff like this doesn't happen."
This thought crossed Jeff Bauman's mind after he noticed a black backpack on the ground near the finish line of the 2013 Boston Marathon.
It made him suspicious, he told a jury on the second day of Dzhokhar Tsarnaev's trial. "I thought it was weird. If you are at the airport, if you see any unattended luggage, you notify authorities."
But this was Boston.
Bauman nonetheless told a friend that they should move; the explosion came two seconds later.
"I saw a flash, heard like three pops and I was on the ground," Bauman said from the witness stand Thursday. "At first I opened my eyes and saw the sky. The first thought was, that was a big firework."
Three people were killed and more than 260 injured when a pair of pressure cooker bombs exploded. A fourth person, an MIT police officer, was ambushed and killed in his patrol car three days after the bombings as Tsarnaev and his brother, Tamerlan, allegedly ran from police.
Tamerlan Tsarnaev, 26, was killed after a gunbattle with police.
Dzhokhar Tsarnaev is now on trial, charged with 30 counts related to the bombings.
Tsarnaev's attorneys admit that he was carried out the attacks but argue that his participation was influenced by his slain brother.
'I could see my bone'
Bauman's ears were ringing, and everything was muffled, but he heard the screams.
The first bomb had exploded.
"I looked down and saw my legs, and it was pure carnage," he told jurors. "I could see my bone."
He kept repeating to himself, "This is messed up, this is messed up, this is messed up."
Then the second explosion.
"We are under attack," he thought to himself and all he wanted to do was call his mom.
Bauman testified about becoming aware of his injuries -- burns, wounds on his back, and his legs.
"I knew my legs were gone. I know that instantly," he said.
Several other survivors took the stand a day earlier, recounting their terror in the aftermath of the bombings.
With Bauman, as with the previous day's survivors, the defense opted not to cross-examine them.
CNN's Ann O'Neill reported from Boston and Mariano Castillo wrote the story in Atlanta.
TM & © 2015 Cable News Network, Inc., a Time Warner Company. All rights reserved.